Today, liquid crystal display devices (LCDs) are widely used as flat panel displays (FPDs) due to their low weight and small power consumption.
However, because the LCD is a light-receiving device, not a self-emitting display, and due to technical limitations with respect to brightness, contrast, viewing angle and size, efforts are underway to develop a new flat panel display that may overcome the shortcomings of the LCD.
An organic electroluminescence device, a new type of flat panel display, is a self-emitting device, has a good viewing angle and contrast compared with the LCD, and can be lightweight, thin and advantageous in terms of power consumption because it does not require a backlight.
In addition, the organic electroluminescence device can be driven at a low DC voltage, has a fast response speed, is resistant to external impacts because it is entirely solid, has a wide usage temperature range and incurs a low fabrication cost.
In particular, in contrast to the LCD or a plasma display panel (PDP), a fabrication process of the organic electroluminescence device is quite simple and entails using only deposition and encapsulation equipment.
In the related art, a passive matrix type driving method that does not have thin film transistors (TFTs) is commonly employed to drive the organic electroluminescence device.
However, in the passive matrix type driving method, elements are formed in a matrix that scan lines and signal lines cross, so in order to drive each pixel, the scan lines should be sequentially driven over time.
In order to obtain a desired average luminance, an instantaneous luminance obtained by multiplying the number of lines by an average luminance should be obtained.
Accordingly, in this method, as the lines increase in number, a higher voltage and more current may be instantaneously applied, accelerating degradation of elements and increasing power consumption, so passive matrix electroluminescent display technology may not be suitable for high resolution large-scale displays.
However, in an active matrix display, the TFTs for switching each pixel are positioned at each pixel.
A first electrode of the TFT turns on or off each of the pixels, and a second electrode facing the first electrode is used as a common electrode.
In the active matrix display, voltage applied to a pixel is charged in a storage capacitor to apply power until a next frame signal is applied, so it may be continuously driven during one screen regardless of the number of scan lines.
Accordingly, with the active matrix display, even though only a low current is applied, the same luminance as with the passive matrix display may be obtained with low power consumption, good image resolution (minuteness or fineness) and increased size.
Characteristics of a basic structure and operation of the active matrix type organic electroluminescence device will now be described.
FIG. 1 shows the basic pixel structure of a general active matrix type organic electroluminescence device.
As shown in FIG. 1, scan lines G are formed in a first direction on a substrate, and signal lines D and power supply lines P2 are formed separately at certain intervals in a second direction perpendicular to the first direction.
A region formed where the scan line G and the signal line D cross is defined as a single pixel area.
A switching TFT Ts, an addressing element, is formed near the crossing of the scan line G and the signal line D.
A storage capacitor Cst is formed to be connected with the switching TFT and the power supply line P2.
A driving TFT Td, a current source element, is formed to be connected with the storage capacitor Cst and the power supply line.
An organic electroluminescent diode (OELD) is connected with the driving TFT Td.
When current is supplied in a forward direction to an organic light emitting material of the organic electroluminescent diode (OELD ), electrons and holes are moved through a P (positive)—N (negative) junction part between an anode (anode electrode), which serves as a hole providing layer, and a cathode (cathode electrode), which serves as an electron providing layer, and they recombine to move from a higher energy to a lower energy state. Accordingly, the organic electroluminescent diode uses the principle that light is emitted due to the energy difference.
In this case, the switching TFT Ts serves to control voltage and store the current source.
The driving principle of the active matrix type organic electroluminescent (EL) device will now be described.
In the active matrix type organic EL device, when a signal is applied to a corresponding electrode according to a selection signal, a gate of the switching TFT is turned on and a data signal passes through the gate of the switching TFT so as to be applied to the driving TFT and the storage capacitor.
When a gate of the driving TFT is turned on, current is applied from the power supply line through the gate to illuminate an organic electroluminescent layer.
The opening/closing degree of the gate of the driving TFT differs according to a size of the data signal, so by controlling the amount of current flowing through the driving TFT, a gray scale can be represented.
During a non-selection interval, data charged in the storage capacitor is continuously applied to the driving TFT to continuously illuminate the organic EL device until a signal of a next screen is applied.
Based on this principle, in the active matrix type organic EL device, a low voltage and an instantaneously low current compared with the passive matrix type organic EL device can be applied.
In addition, the active matrix type organic EL device can be continuously driven during a single screen time regardless of the number of selection lines, and thus it may allow a low power consumption, high resolution and large area.
The active matrix type organic EL device operates by current flow through TFTs.
In the related art, amorphous silicon (a-Si) TFTs have a low electric field effect mobility, so polycrystalline silicon (polysilicon or p-Si) TFTs with good electric field effect mobility and uniform grains are required.
Polysilicon TFTs can generate a driving circuit on the substrate with a high electric field effect mobility, so by directly forming a driving circuit with the polysilicon TFTs on the substrate, a cost for a driving IC can be reduced and mounting may be simplified.
A method for fabricating TFTs by using the related art organic EL device fabrication method will now be described with reference to FIGS. 2A to 2E.
FIGS. 2A to 2E are process sectional views showing a method for fabricating a TFT according to the related art.
Referring to FIG. 2A, a metallic material is deposited on the substrate 11 and selectively patterned through exposing and developing processes by using photolithography to form a gate electrode 13 on the substrate 11.
Next, referring to FIG. 2B, a gate insulation film 15 and an active layer 17 are sequentially deposited on the entire surface of the substrate including the gate electrode 13.
To deposit the active layer 17, PECVD equipment, which is generally expensive, or other deposition equipment can be used.
And then, referring to FIG. 2C, the active layer 17 is selectively patterned through the exposing and developing process by using photolithography to form an active layer pattern 17a. 
Thereafter, with reference to FIG. 2D, a metallic conductive layer 19 is deposited on the entire surface of the substrate including the active layer pattern 17a. 
With reference to FIG. 2E, the metallic conductive layer 19 is selectively patterned through the exposing and developing process using photolithography to form source and drain electrodes 19a and 19b, thereby completing fabrication of the TFT.
However, according to the related art, because expensive PECVD equipment is used to form the active layer of the TFT structure, material costs and processing time increase and productivity drops.